Why those an apple become brown after sometime

Enzymes in Apples

The cells of apples, and many other fruits and vegetables, act as balloons, holding enzymes ( a protein that increases the rate of chemical reaction) inside. Portions of the cells contain phenolic compounds, and other parts house enzymes. These enzymes, known as polyphenol oxidase enzymes, are found in the chloroplasts of the cells. When the cells remain intact, as when the apple stays whole, the fruit keeps its normal coloring, but cutting the fruit and exposing it to air changes this.

Oxygen Invasion

Anything that breaks the walls of an apple allows the polyphenol oxidase enzymes and the phenolic compounds to come together. When air enters the mixture, oxidation occurs, producing o-quinones. These mix with amino acids or proteins to create the browning seen on the cut surface of an apple.